An optical disc is characterized by its high data density, large data capacity and high speed data access, and various research and development programs are presently in progress. For a recording medium of one type of optical disc which allows an additional recording only once, TeOx, TeC, Te-Sn-Se and the like are known in the art and some of them have been already marketed.
With an optical disc dedicated to playback, the playback on both sides of the disc is realized by using a disc wherein two recording layers each having a reflection layer of Al are bonded to each other with a hot-melt adhesive as disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 6538/1983. An optical disc with which information is recorded by providing microscopic holes (known as pits) on a recording layer with a laser beam has an air-sandwich structure, as shown in FIG. 4, with which the adhesion is provided on the inner and outer circumferencial areas of a disc where a recording layer 1 does not exist. In such an example, a heat-hardening type epoxy adhesive or an ultraviolet-ray-hardening adhesive is used as an adhesive 6 to bond both supports 4 and 5 to each other via spacers 2 and 3 to enclose air space 7. Other than these adhesives, a two-liquid type normal-temperature-hardening epoxy adhesive may be also used. At the same time, as an erasable optical recording system, the magneto-optical recording system has attracted attention.
Some of magneto-optical recording media, for example, an optical disc, involve transparent supports on which a magnetic alloy layer composed of at least one of rare earth elements such as Sm, Eu, Gd, Tb, Dy, Ho, Er and the like as well as at least one of transition elements such as Fe, Co, Ni and the like is formed through sputtering or vacuum depostion to form an amorphous layer, whereupon a protective layer comprising organic substances may be coated in order to provide wear resistance.
With an magneto-optical disc, there is no need for the air sandwich structure, mentioned above. Accordingly, it is advantageous in terms of cost to bond the whole surface with the counterpart.
However, such a magneto-optical recording medium has as a recording layer a very corrosive material for example Gd, Tb or Fe. When two recording layers are bonded to each other by the above-mentioned hot-melt adhesive, the layers are readily corroded with such an adhesive. Therefore, such an adhesive is unusable for magneto-optical disc. Additionally, the recording layer transmits only a little amount, if any, of ultraviolet rays. Consequently, an ultraviolet-ray-hardening adhesive cannot be used to mutually bond two supports each having a recording layer. Additionally, such supports having a recording layer are hereinafter called recording elements. When an epoxy adhesive is used, a two-liquid-mixing type of the similar adhesive involves a complex process control, and, furthermore, insufficient mixing of two liquids may cause corrosion of the layer, because the hardener itself corrodes Gd, Tb and Fe. With a mono-liquid heat-hardening type of the similar adhesive, it is difficult to use a plastic for the recording element because of the heat deformation. Also, it is impossible to avoid thermal degradation of the recording layer. The production cost is higher, as this type of adhesive necessitates the provision of hardening furnace.
Though a mono-liquid-hardening type polyurethane adhesive and the like are useful adhesives as their mono-liquid non-solvent component hardens at normal temperature range, without corroding a recording layer. However, with this type of adhesive, the hardening process is slower and takes more than one day if it is left in an ordinary room, resulting in the poor productivity.
Additionally, it is technically difficult to apply a fast-hardening adhesive, such as a cyanoacrylate adhesive, upon the larger area such as the surface of disc, and the adhesive itself is expensive.